<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .active1 {
            background: url(./images/1.jpg) no-repeat center/500px 500px;
        }
        .active2 {
            background: url(./images/2.jpg) no-repeat center/500px 500px;
        }
    </style>
</head>
<body>
    <button>皮肤1</button>
    <button>皮肤2</button>
    <script>
        var btnEle = document.querySelectorAll("button") ;
        btnEle.forEach(function( item , key ){
            item.onclick = function (){
                if ( key == 0 ) {
                    document.body.className = "active1" ;
                } else if ( key == 1 ) {
                    document.body.className = "active2"
                }
            }
        }) ; 
    </script>
</body>
</html>